Alumina (Al.sub.2 O.sub.3) has been a favored ceramic substrate material for electronic circuitry where long life and high reliability are required. However, for certain high performance applications, sintered alumina substrates have proven less than satisfactory.
While an alumina body can be ground to a smooth finish, it undergoes high shrinkage (about 18%) on firing. Sintered alumina has a relatively high dielectric constant of about 10, which restricts line spacing and may lead to signal delays and noise in operation. A relatively high coefficient of thermal expansion (about 65.times.10.sup.-7 /.degree.C.), as compared to that of silicon chips (about 35.times.10.sup.-7 /.degree.C.), can make it difficult to seal such chips to an alumina substrate. Finally, high firing temperatures (about 1600.degree. C.) are required for co-sintering. This limits the metals that may be employed to molybdenum and tungsten, and excludes silver, copper and gold.
Accordingly, a search has been made for substrate materials having more compatible properties than sintered alumina. In particular, materials having coefficients of thermal expansion more closely matched to silicon, and a lower dielectric constant than alumina, have been sought.
Glass-ceramic materials, in particular the cordierite-type, have received considerable attention. Some of the history is reviewed in co-pending application Ser. No. 07/238,574 filed Aug. 31, 1988, and assigned to the assignee of this application. This co-pending application discloses modified cordierite compositions that have superior properties to those of alumina, and that can be sintered below 1000.degree. C. The latter property permits co-sintering, that is, firing metal circuitry and sintering the substrate in one heat treatment.
Aluminum nitride (AlN) is another ceramic material that holds considerable promise as a substrate material. In addition to a compatible coefficient of thermal expansion, this material has an exceptionally high thermal conductivity. This permits dissipation of heat from soldering and sealing operations that might otherwise damage the circuitry. U.S. Pat. No. 4,719,187 (Bardhan et al.) describes AlN and an improved method for its production.
The advent of these lower expansion substrate materials has necessitated development of new metallizing pastes, particularly for application of thick film circuitry to presintered substrates. In general, electroconductive metals can be applied as thin films where interior circuitry is involved, or where the circuitry and a "green" substrate are to be co-sintered.
The term "thin film" refers to a suspension of metal powder in a vehicle without any additive. In contrast, the term "thick film" refers to a suspension of metal powder containing a vitreous additive. The latter provides adhesion to the substrate when the paste is fired. The present invention is concerned with thick film pastes containing a vitreous additive. Prior commercial metal pastes, such as palladium-silver (Pd-Ag) and copper pastes, contained additives adapted to seal to alumina substrates. It has been found that these pastes do not adhere well when fired on the new lower expansion materials.